1. Field of the Invention
The present invention relates generally to testing circuits for glass-break sensors and more particularly to a testing circuit for a glass-break sensor in which the sensor is not disabled during a self-test and which utilizes a test sound at a frequency that is normally used by the sensor for the detection of breaking glass.
2. Description of the Related Art
Audio intrusion detection systems that detect the audio characteristics of breaking glass are well-known in the art. The simplest such systems use a microphone to detect the audio sound produced by the breaking glass and a threshold circuit to determine whether one or more of the frequency components of the sound exceed a predetermined threshold that is characteristic of breaking glass.
More complex glass-break detectors include timing/ comparison logic that compares the sound created by the microphone to the time-varying audio characteristics of breaking glass. These types of detectors isolate at least two frequency components of the audio sound and signal an alarm if the sound corresponds to the time-varying function, i.e., if certain frequencies are received at predetermined times and for predetermined durations.
For example, Petek, U.S. Pat. No. 5,323,141, concerns a glass-break sensor that includes a pair of microphones. Each microphone is used to detect one of the characteristic components of an acoustic wave generated by a glass-break. One of the microphones is used to detect a low-frequency signal and the other to detect a high-frequency signal.
Marino et al., U.S. Pat. No. 5,117,220, concerns a glass-break detector that detects structurally-transmitted vibrations and airborne sounds indicative of breaking glass. This system detects a low-frequency signal at about 200 Hz and a high-frequency signal at about 3-7 kHz. These signals are detected in accordance with a time-dependent function to provide an indication of breaking glass.
Smith et al., U.S. Pat. No. 5,192,931, concerns a glass-break detector that includes a low-frequency channel for detecting inward flex of a breaking window and a high-frequency channel for detecting the acoustic characteristics of breaking glass. The two channels are combined in a logic circuit that is timed so that the low-frequency flex is detected initially with detection of the high-frequency component following shortly thereafter. If both timing conditions are fulfilled, an alarm signal is generated. The low-frequency channel detects signals in the range of 50-100 Hz, and the high-frequency channel detects signals over a range of high frequencies.
Rickman, U.S. Pat. No. 5,164,703, concerns a supervisory circuit for use with an audio intrusion detection system. The system includes a first detector for detecting a low-frequency signal in the range of 3-30 Hz. Once the low-frequency signal has been detected, a circuit for detecting a higher frequency signal in the range of 7-16 kHz is enabled.
Davenport et al., U.S. Pat. No. 4,668,941, concerns a glass-break sensor that detects a low-frequency signal in the range of 350 Hz and a high-frequency signal in the range of 6.5 kHz. To generate an alarm signal, the low-frequency component must be detected first, with the high-frequency component detected a short time thereafter.
These patents exemplify various techniques for detecting the time-varying audio characteristics of breaking glass, including the use of different frequency components and different time-varying functions to model breaking glass.
Systems for testing glass-break sensors are also well-known in the art. In a typical testing system, the alarm triggering mechanism or the sensor triggering mechanism is disabled while a test signal is generated or an intrusion is simulated. Once the test is complete, the alarm trigger or sensor trigger is re-enabled.
The aforementioned U.S. Pat. No. 5,164,703 concerns a glass-break sensor in which the detection of a low-frequency signal is used to enable the detection of a high-frequency signal. If the high-frequency signal is then detected, an alarm is generated. A supervisory test circuit includes a self-test timer that initiates a self-test if the low-frequency signal is not received within a predetermined self-test time, preferably 19 hours. During the test, the high-frequency detection line is partially disabled while a test sound is generated. If the test sound is received by the microphone, the self-test timer is reset. If the test signal is not detected, an error signal is generated. Because the high-frequency detection line is partially disabled while the test sound is being generated, if a glass-break were to occur during the test, an alarm would not be generated. This is obviously disadvantageous in that the sensor could be defeated by breaking the glass while a test is taking place.
U.S. Pat. Nos. 3,022,496, 3,134,970, 3,487,397, 3,928,849, 3,974,489 and 4,386,343 concerns other types of testing systems for alarms. In each of these patents, the detection system is also disabled during a self-test. This renders each of these systems vulnerable to undetected break-in during testing.
Spies et al., U.S. Pat. No. 4,950,915, concerns an impact sensor for a vehicle. The impact sensor includes an acceleration sensor for detecting crash sounds, an evaluating circuit, and a trigger circuit for releasing an air-bag. For testing the impact sensor, an electro-acoustic transducer is provided in the sensor housing. During a self-test, the electro-acoustic transducer emits a test signal that is received by the acceleration sensor. The electrical signals that are produced by the acceleration sensor during the test are evaluated by a testing circuit to ensure that the acceleration sensor is in proper working condition. In a preferred embodiment, during testing the trigger circuit is disabled. In this embodiment, the air-bag, which is usually triggered by the impact sensor, could not be activated during a self-test. In an alternative embodiment, testing of the impact sensor may occur with the trigger circuit activated by selecting a test sound that is different from the sound necessary to activate the trigger circuit. This embodiment of the device enables the trigger circuit to remain active during a test but the acceleration sensor cannot be tested at a frequency at which it normally detects crash sounds.
Thus, there is a need for a glass-break sensor testing circuit that is not disabled during a self-test so that the system is not vulnerable to break-in during a test. It would be highly desirable to have such a glass-break sensor utilize a test sound at a frequency normally used by the sensor for detecting breaking glass.